Minimum Safe Flow For Centrifugal Pump

Minimum Safe Flow for pumps has been discussed on numerous occasions on "Cheresources". Many posts have discussed the line sizing, flow control and the scheme for common minimum flow piping from multiple pumps operating in parallel.

Minimum Continuous Stable Flow: It is the lowest flow at which the pump can operate without exceeding acceptable vibration limits.

Minimum Continuous Thermal Flow: It is the lowest flow at which the pump can operate without its operation being impaired by the temperature rise of the pumped liquid.

Pump operations below these points can cause shaft vibration and / or reduce the mechanical seal life both due to vibrations and high fluid temperatures.

To avoid such problems, high capacity pumps shall be provided with a minimum flow bypass with the flow controlled by a restriction orifice or a control valve. If a restriction orifice is used, the operating flow rate of the pump should be increased to account for the continuous bypass flow.

The sizing of the minimum bypass flow circuit shall be based on the higher of the two minimum flows.

If it is not possible to provide a minimum flow circulation line, then adequate instrument protection shall be provided to prevent pump operation below the minimum allowable flow rate.

Today's blog entry is more about "minimum continuous thermal flow". When there is no flow from the pump by way of a valve closed to the pumped liquid reciever the power input to the pump is converted to heat in the casing of the pump causing the temperature of the liquid to rise in the casing. To prevent this happening the "minimum continuous thermal flow" needs to be ensured through the pump when it is operating.

Most experienced process engineeers are aware that this minimum safe flow to prevent temperature rise is provided by the pump vendor. During the preliminary phase of engineering, when no vendor data is available for the minimum safe flow and the minimum safe flow recirculation line is to be sized an approximation of the minimum safe flow needs to be done. Often, this assumed or approximated value is 25 to 30% of the forward flow. Obviously after the receipt of the vendor data for the minimum safe flow, the recirculation line hydraulics needs to re-checked based on the value provided by the pump vendor.

However, it is possible to calculate the minimum safe thermal flow for general purpose centrifugal pumps based on the spreadsheet that I am attaching. However, this spreadsheet is only applicable for existing pumps where data such as brake horse power at shut-off, shut-off head and the pump efficiency at shut-off is available. In other words, the spreadsheet may be used to verify the minimum safe flow given by the pump vendor. The equations for calculating the "allowable temperature rise" and the "minimum safe thermal flow" are in USC units. However, I have programmed the spreadsheet to take inputs in SI units and give the output of minimum safe flow in both USC units (US gpm) and Metric units (m3/h).

Hope all of you find this blog entry and the accompanying spreadsheet interesting and useful. I would be happy to receive your comments and will try to answer any queries raised by you.

I have condition here, for closed cooling water circulating pump having total flow rate 1240M^3 /hr but i want to rum the pump in a closed loop for chemical cleaning and total volume of my system is approximately 90M^3. I have minium re circulation line also between suction & discharge common header. We are by passing heat exchanger and supply to all system just we need to circulate the water to clean the line.
My question is:
Can i rum the pump with is much quantity of water ?

I have condition here, for closed cooling water circulating pump having total flow rate 1240M^3 /hr but i want to rum the pump in a closed loop for chemical cleaning and total volume of my system is approximately 90M^3. I have minium re circulation line also between suction & discharge common header. We are by passing heat exchanger and supply to all system just we need to circulate the water to clean the line.My question is:Can i rum the pump with is much quantity of water ?

Chemical cleaning using minimum flow recycle line is not something new and is frequently employed, However, if there is a specific flow requirement for cleaning then there should be some flow control device in the recycle line such as a control valve or a flow control restriction orifice.

Thank you ankur for your post. I developed a sizing sheet for single phase flow lines based on API 14 E and received comments. To implement these comments i need advanced pogramming (VBA). Till now i can't find guidelines or references to do that.

Could u help me find such resources to be able to program my sheet properly?

What is Meant by efficiency at shut off? At pump shut off head, the efficiency will be zero

Ayush,

Your statement is incorrect. The pump efficiency will be low but it cannot be zero. The pump is still running against a blocked discharge and the mechanical energy from the driver of the pump is being lost by generating heat in the pump casing instead of overcoming the head as change in the potential energy of the liquid being pumped

Your spreadsheet would be even more useful if you added calculations that provide the maximum time that a pump could be run at flowrates less than the calculated minimum safe flow (i.e., the time it would take to heat up the fluid enough to reach the vapor pressure)

if you say minimum recycle line shall approx be 25 to 30 % of the forward flow line but i have seen in one P&ID as under :

discharge line ( forward line ) for pump is 3 inches

minimum recycle flow line is also 3 inches ( same as forward flow line)

my inderstanding is that is must be lesser than forwad flow line ( discharge ) of pump.

2ndly there is SDV on the combine header of the dieschage lines of parralel pumps . if SDV shuts down , the maximum flow will flow back through reclyle line of each pump therefore minimum recycle line is same as forward line ( discharge ) of pump .